1. Field of the Invention
The present invention relates to a process for purifying wet process phosphoric acid. More particularly, it relates to a process for producing phosphoric acid having high purity which is suitable for industrial uses and food additive uses etc.
2. Description of the Prior Art
The wet process phosphoric acid prepared by treating phosphate rock with sulfuric acid and separating calcium sulfate usually contains impurities such as iron, aluminum, sodium, calcium, magnesium titanium, sulfuric acid, hydrofluoric acid and silica which are derived from the raw materials, phosphate rock and sulfuric acid. Recently, wet process phosphoric acid has been used not only for the preparation of fertilizer but also for industrial uses and food additive uses after separating the impurities.
Various processes for purifying the wet process phosphoric acid have been proposed. The solvent extraction process has been the process in practice used. In the solvent extraction process, phosphoric acid is extracted from the wet process phosphoric acid with organic solvents such as alcohols e.g. n-butyl alcohol, iso-butyl alcohol, isoamyl alcohol, cyclohexanol; ketones e.g. methyl ethyl ketone, methyl isobutyl ketone; ethers e.g. isopropyl ether and phosphoric acid esters e.g. tributyl phosphate, etc. and then, phosphoric acid is back-extracted with water.
In these processes, the impurities in the wet process phosphoric acid are usually in the form of phosphates. The phosphates are not usually extracted by the organic solvent and are easily precipitated as the phosphates depending upon the decrease of the phosphoric acid concentration whereby the scaling is caused in extractor. Accordingly the phosphoric acid extraction efficiency is limited by these circumstances. Thus, 50 to 70 wt.% has been the maximum phosphoric acid extraction efficiency. Accordingly, P.sub.2 O.sub.5 in the residual aqueous solution in the solvent extracting process has to be used as fertilizer or the raw material for producing salts where high purity is not required.
Various processes for improving phosphoric acid extraction efficiency have been proposed. In the practically used process, hydrochloric acid is included in the extraction system. In the process, the phosphoric acid extraction efficiency is relatively improved whereas the purification efficiency in removing certain impurities is decreased depending upon the increase of hydrochloric acid in the extraction system. When excess hydrochloric acid is included in the extraction system, the zinc component in the wet process phosphoric acid is extracted along with the phosphoric acid into the organic solvent phase, and accordingly, zinc contaminates the purified phosphoric acid. It has been proposed to discharge a part of the zinc-containing solution from the extraction system while maintaining the concentration of hydrochloric acid in the extraction system lower than a certain level in order to prevent contamination with zinc (Japanese Unexamined Patent Publication No. 17399/1974). In this process, the operating sections and the control of the operation are complicated and the concentration of hydrochloric acid in the acid extracting section has to be lowered whereby satisfactory phosphoric acid extraction efficiency cannot be attained.
Recently, from the viewpoint of the increased cost of phosphate rock and the need to treat the waste solution containing phosphoric acid or phosphate to control pollution, it is necessary to improve the phosphoric acid extraction effeciency. Thus, the phosphoric acid extraction efficiencies of about 95% are not satisfactory and substantial amounts of phosphoric acid should be extracted.
The residual aqueous solution in the solvent extraction process contains most of the impurities included in the wet process phosphoric acid and has a remarkably low concentration of phosphoric acid. Therefore, it is difficult to use the residual aqueous solution for the preparation of fertilizer and the phosphoric acid is usually lost.
From the viewpoint of control of environmental pollution caused by phosphoric acid, the residual aqueous solution cannot be directly discharged. Accordingly, it is necessary to recover the phosphoric acid component from the residual aqueous solution or to solidify the phosphoric acid component as insoluble apatite. The decrease of phosphoric acid extraction efficiency causes the increase of costs caused by the loss of phosphoric acid remaining in the residual aqueous solution and the treatment of the residual aqueous solution.
The inventors have researched a process for producing purified phosphoric acid containing no zinc by extracting and purifying the wet process phosphoric acid in the presence of enough hydrochloric acid to extract a substantial amount of the phosphoric acid from the wet process phosphoric acid. As a result, the inventors have found that purified phosphoric acid containing no zinc can be obtained by contacting phosphoric acid containing zinc which is obtained by the solvent extraction process in the presence of hydrochloric acid with an anion-exchange resin.
As is well-known, when excess chloride ions are present, zinc forms chloro-complex ions which are adsorbed on the anion-exchange resin. It has been believed that more than 35 g/liter preferably 70 g/liter as Cl is required, in order to convert the zinc to the chloro-complex ions for adsorption on the anion-exchange resin. However, it has been found that the zinc can be completely adsorbed on the anion-exchange resin even in the presence of a remarkably low concentration of chloride ions when phosphoric acid is included at a concentration of 20 to 60 wt.% as P.sub.2 O.sub.5. The inventors have found that satisfactory adsorption of the zinc component could be attained in the concentration range of 20 to 60 wt.% as P.sub.2 O.sub.5 of phosphoric acid and 10 to 40 g/liter as Cl of chloride ion. When the chloride ion concentration is more than 40 g/liter especially more than 70 g/liter as Cl, substantial adsorption of the zinc component does not take place. Even though the phosphoric acid containing zinc has a phosphoric acid concentration and chloride ion concentration in the above-mentioned range, when the zinc content is varied, the amount of the anion-exchange resin, the contacting conditions and the control of the operation have to be varied depending upon the zinc content, whereby it is difficult to completely remove the zinc in industrial practice. Accordingly, it is necessary to maintain a substantially constant zinc content in the phosphoric acid treated in the operation for removing the zinc by the anion-exchange resin. This fact has been also found by the inventors.
The inventors further researched a process for producing purified phosphoric acid which is suitable for industrial uses and food additive uses by extracting and purifying wet process phosphoric acid in the presence of hydrochloric acid to obtain a phosphoric acid which contains substantial amounts of zinc but does not substantially contain the other impurities (hereinafter referred to as extracted phosphoric acid) and then, removing the zinc component from the extracted phosphoric acid by an anion-exchange resin.
As a result, it has been found that under extraction conditions yielding less than a certain hydrochloric acid concentration in the extracted phosphoric acid, the zinc content in the extracted phosphoric acid has varied widely or the desired phosphoric acid extraction efficiency has not been obtained or the desired purity of phosphoric acid has not been obtained or scale has been formed in the extractor whereby there has been no success in obtaining extracted phosphoric acid which contains a constant zinc concentration and is substantially free from other impurities at a high phosphoric acid extraction efficiency.
The inventors have discovered the problems to be overcome by novel consideration and have further found that the partition coefficient of chloride ions in the extraction system varies depending upon the impurities and the phosphoric acid concentration and accordingly, the chloride ion concentration in the entire extraction system is highly variable in the conventional process and the partition coefficient of zinc is highly dependent upon the variation of the chloride ion concentration and accordingly, the movement of zinc is so complicated that a large amount of zinc accumulates in the extraction system, and the phenomenon of zinc accumulation in the extraction system is the main cause of the trouble. The present invention has been attained as a result of this finding.